As the worldwide demand for fuel increases, interest in sources other than crude oil from which to produce transportation fuels, including aviation fuels, is ever increasing. For example, due to the growing environmental concerns over fossil fuel extraction and economic concerns over exhausting fossil fuel deposits, there is a demand for using an alternate or “green” feed source for producing hydrocarbons for use as transportation fuels and for use in other industries. Such sources of interest include, for example, biorenewable sources, such as vegetable and seed oils, animal fats, and algae byproducts, among others as are well-known to those skilled in the art. A conventional catalytic hydro-processing technique is known for converting a biorenewable feedstock into green diesel fuel that may be used as a substitute for the diesel fuel produced from crude oil. As used herein, the terms “green diesel fuel” and “green jet fuel” refer to fuel produced from biorenewable sources, in contrast to those produced from crude oil. The process also supports the possible co-production of propane and other light hydrocarbons, as well as naphtha or green jet fuel.
Biomass fermentation products typically include lower isoalkanols such as, for example, C3 to C8 isoalkanols obtained by contacting biomass with biocatalysts that facilitate conversion (by fermentation) of the biomass to isoalkanols of interest. The biomass feedstock for such fermentation processes can be any suitable fermentable feedstock known in the art, such as fermentable sugars derived from agricultural crops including sugarcane, corn, etc. Suitable fermentable biomass feedstock can also be prepared by the hydrolysis of biomass, for example lignocellulosic biomass (e.g. wood, corn stover, switchgrass, herbiage plants, ocean biomass, etc.), to form fermentable sugars.
Jet-range fuels are an important product for the aerospace industry and the military. The specific characteristics of various grades and types of jet-range fuels vary slightly according to the particular application and environment in which they are used. Generally, jet-range fuels comprise a mixture of primarily C8 to C16 hydrocarbons and typically have a freezing point of about −40 or −47° C. (−40 or −52.6° F.). In order to produce jet-range fuels from isoalkanols derived from fermented biomass, in one example known in the art, isobutanol is first dehydrated to form butenes. The butenes are then oligomerized, in the presence of an oligomerization catalyst, in one or more reactors to form heavier olefins, such as C5 to C20, or even higher, olefinic oligomers. Finally, the resulting olefinic oligomers are hydrogenated in a saturation reactor to form the corresponding C5 to C20, or even higher, paraffins in a mixture which can then be subjected to separation to obtain C9 to C20+ paraffins suitable for use as biorenewable jet fuel.
Since the oligomerization reaction is highly exothermic, the butene fed to the oligomerization reactors may be cooled before entering the oligomerization reactors. Another measure taken to control the temperature increase in the oligomerization reactors is to limit the proportion of olefins contained in the feedstream provided to each reactor to no more than about 15 percent by weight (wt %). This is accomplished, at least in part, by adding non-reactive diluent material to the reactors which also provides a heat sink to control the temperature rise in the reactors.
Typically, this dilution may be done by recycling saturated distillate product from a stripped effluent of a hydrogenation section back to the oligomerization and hydrogenation reactors. The saturated recycle is, for the most part, inert across the hydrogenation reactor. Nevertheless, while the diluent does control the temperature, it may impose limitations on the processing.
Therefore, it would be desirable to have one or more processes which efficiently and effectively dilute a feedstock to an oligomerization reactor.